What is Atomic Absorption Spectroscopy (AAS)

Atomic absorption spectroscopy, or AAS, is a technique for measuring the concentrations of metallic elements in different materials.

As an analytical technique, it uses electromagnetic wavelengths, coming from a light source.

Distinct elements will absorb these wavelengths differently. It gives a picture of what concentrations of a specific element there is in whatever material, or liquid, is being tested.

Here we look at what AAS involves as an analytical technique, what it can measure, why it is useful, and the instruments involved in carrying it out.

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What is Atomic Absorption Spectroscopy?

Spectroscopy is the study of how radiated energy and materials interact. Matter absorbs energy, which will create some sort of change in its state.

The atomic part refers to the atoms in a material, which will absorb radiated energy from a light source.

These atoms will each have their own characteristics when it comes to absorbing energy because each element has a unique electronic structure.

Therefore, using AAS, you can measure for a specific element in a material, based on the amount of light absorbed at a defined wavelength, which corresponds to the known characteristics of the element you are testing for.

Typical calibration curve for an element measured by AAS
Typical calibration curve for an element measured by AAS

Note: AAS is also referred to as atomic absorption spectrometry. The difference between spectroscopy and spectrometry is that spectroscopy is the study of how energy and materials interact, while spectrometry refers to how you apply this as a measuring technique. For practical purposes, it doesn’t really make any difference which term you use.

When was Atomic Absorption Spectroscopy first used?

As a phenomenon, atomic absorption spectrometry was first discovered in 1802, when the English scientist William Hyde Wollaston observed and described dark lines in the sun’s spectrum.

In 1817, the German physicist Josef von Fraunhofer carefully mapped out these spectral absorption lines, which are now named after him.

A theory of spectrochemical analysis then developed with the work of the scientists Gustav Kirchhoff and Robert Bunsen in 1860.

Kirchhoff and Bunsen developed the spectroscope, splitting light into wavelengths.

It was not until the 1930s that this technique became more widely used.

However, atomic absorption spectroscopy as a modern technique for chemical analysis dates from 1955, when the Lancashire-born scientist Alan Walsh published his significant paper on the potential for AAS in Melbourne, Australia.

Walsh’s breakthrough came with the realisation that he needed to be measuring absorption of light rather than emission.

This led to the development of new techniques for AAS. The first commercially available instruments appeared in the 1960s.

As AAS has developed since this time, with the continuing application of new technology, including automation and computers, it has become an extremely reliable analytical technique.

It is fast, sensitive, specific and user-friendly.

Why should I use Atomic Absorption Spectroscopy?

AAS provides a high degree of accuracy.

Normally results fall within a range of 0.5 per cent to 5 per cent accuracy, but this may improve further depending on the standards set for testing and analysis.

It is a highly sensitive method of analysis.

In a given material, it can measure parts per billion of a gram.

In applications such as medicine and pharmaceuticals, AAS has helped revolutionise practices, detecting things such as trace toxins.

In some sectors, this method has been able to detect elements which people were previously unaware existed in certain material, such as cobalt and molybdenum in soil.

It is a technique that is well-suited to reaching otherwise inaccessible places, such as mines, to test rocks to see if they are worth mining.

Modern AAS systems are a comparatively inexpensive means to accurately detect specific elements.

When should I use Atomic Absorption Spectroscopy?

Atomic absorption spectroscopy has different laboratory and testing applications in industrial, clinical and research settings, as a crucial component in various processes.

These processes include:

  • Quality control
  • Toxicology
  • Environmental testing

As a method, AAS can analyse the content of certain metals in various materials.

  • In environmental testing, it can measure the concentration of various elements in rivers, drinking water and seawater.
  • In the food and drink industry, it can measure various concentrations of elements in wine, beer and fruit drinks and test for types of contamination in food.
  • For pharmaceutical companies, AAS can determine the minute quantities of catalyst materials used in the manufacture of drugs, and for other impurities.
  • In industry, there are different raw materials requiring examination and analysis to check they have sufficient amounts of certain major elements, and that there are not too many impurities, some of which could be toxic.
  • In mining, AAS can test for the concentration of valuable materials before excavation operations.
  • AAS is used in agriculture, to analyse plants and soils for mineral content.
  • Metals and other substances can have a bad effect on oil and gas, which is why the petrochemical industry uses AAS.
  • The technique also supports critical testing in the nuclear industry, where there could be potentially hazardous elements in the output of waste and water.

Why is Atomic Absorption Spectroscopy important?

Metals occur naturally in the world around us, and around three-quarters of the earth’s chemical elements are metals.

Sometimes metal content in a material is desirable, but metals can also be contaminants. Therefore, measuring for metallic content is a critical part of many different processes.

Although atomic absorption spectroscopy has been an established method for analysis of materials for metallic elements for many years, it remains a benchmark technique.

This is because it has greater sensitivity than other methods, with less limitations.

For some liquid samples, it can provide direct analysis.

It also works accurately with very small sample sizes, making it rapid, efficient and economical as a testing method.

Various industries and sectors are dependent on this form of testing to ensure their products, or the materials they are processing, are sufficiently free from contamination, or contain the right degree of certain metallic elements to support their intrinsic value.

In clinical analysis, AAS can test for metals in whole blood, plasma, urine, saliva, brain and muscle tissue, the liver and hair.

One example where atomic absorption spectroscopy can provide invaluable support is in measuring mercury levels in fish. Mercury is toxic to humans, but it occurs as a metallic element in the environment. This poses a potential hazard to the food chain.

Mercury can be a pollutant arising from various processes, such as power plants, metal processing and cement production. This mercury air pollution then finds its way into rivers, lakes and oceans.

A process known as biomagnification builds up levels of methyl mercury in predatory fish such as tuna, swordfish and shark. It can also be present in some kinds of shellfish. Methyl mercury is toxic to humans.

Using the advanced capabilities of atomic absorption spectroscopy, fish samples can be tested rapidly and accurately.

AAS is a universally recognised method of analysis across the globe and an important tool in scientific research.

How does Atomic Absorption Spectroscopy work?

Elements exist on an electromagnetic spectrum, and their atoms will absorb wavelengths of light that relate to their particular characteristics.

During the atomic absorption spectroscopy process, these atoms will absorb electromagnetic radiation at a specific wavelength. This produces a measurable signal.

By looking at these signals, it is then possible to determine the parts per million, or ppm, levels of specified metals in the material that is being tested.

What creates these signals?

Within an atom, there are electrons at various energy levels. During the spectroscopy process, the absorbing of energy moves electrons to a more energetic level.

The radiant energy the electrons absorb is directly related to the transition that occurs during this process. The atoms absorb light in an excited state. Atomic absorption measures the amount of light at a resonant wavelength, which passes through a cloud of atoms and is absorbed by them.

Once the excited electrons start to relax again, they emit energy in the form of photons.

Every element has its own unique electronic structure. Therefore, the radiation absorbed represents a unique property of each individual element.

What does Atomic Absorption Spectroscopy measure?

The amount of a specified element present in a material is determined by measuring the amount of light absorbed and the energy emitted during the spectroscopy process.

The changes in these wavelengths of light, before and after absorption, will appear as peaks of energy absorption in a readout.

This is atomic absorption spectroscopy and it is used to measure metal concentration in materials and liquids.

What elements can Atomic Absorption Spectroscopy measure?

An atom will absorb energy through its own specific pattern of wavelengths because it has a unique configuration of electrons.

Atomic absorption spectroscopy can measure known elements in a material based on these unique configurations.

These elements are all metallic. In the periodic table, they are elements which have certain common properties:

  • Metals tend to be good conductors of heat and electricity
  • Most metals have a high density but can be broken up into sheets
  • Most metals corrode if exposed to seawater or air
  • They can be shiny and ductile, which means you can make them into wire
  • They have a solid state at room temperature, except for Mercury
  • Most metals will lose electrons when they react

There are more elements that are metals compared to non-metals, such as oxygen and chlorine.

Here is a list of metallic elements that atomic absorption spectroscopy can measure, with their periodic table symbol shown in brackets:

  • Aluminium (Al)
  • Antimony (Sb)
  • Barium (Ba)
  • Arsenic (As)
  • Beryllium (Be)
  • Cadmium (Cd)
  • Calcium (Ca)
  • Chromium (Cr)
  • Cobalt (Co)
  • Copper (Cu)
  • Gallium (Gu)
  • Hafnium (Hf)
  • Indium (In)
  • Iron (Fe)
  • Lead (Pb)
  • Lithium (Li)
  • Magnesium (Mg)
  • Manganese (Mn)
  • Mercury (Hg)
  • Molybdenum (Mo)
  • Nickel (Ni)
  • Niobium (Nb)
  • Osmium (Os)
  • Potassium (K)
  • Ruthenium (Ru)
  • Silver (Ag)
  • Sodium (Na)
  • Strontium (Sr)
  • Thallium (Tl)
  • Tin (Sn)
  • Tungsten (W)
  • Vanadium (V)
  • Zinc (Zn)
  • Zirconium (Zr)

Why is Atomic Absorption Spectroscopy limited to metals?

Although AAS has been known to work as a measurement technique on some semi-metals, such as boron and silicon, metals work best.

A major reason for this is that the atoms in metal elements are more easily readable.

For AAS to be effective, the atoms in a material must be in isolation and free of possible contaminating lines from molecules.

Metals generally have narrow, single emission and absorption lines, which form brightly and clearly.

This allows for the selective detection that atomic absorption spectroscopy requires.

Is Atomic Absorption Spectroscopy destructive?

The processes involved in atomic absorption spectroscopy mean that it is, ultimately, destructive for the sample.

The sample must be first turned into an atomic gas to then analyse it using AAS.

  • For liquid samples, this involves evaporation
  • Solid samples are vapourised
  • Then any compounds must be broken down into free atoms, and this process is atomisation

However, AAS only requires a small sample size to work.

This can be as little as 10 milligrams, which typically will cause little damage when removed.

How do you perform Atomic Absorption Spectrometry?

To carry out atomic absorption spectrometry, you require three main components:

  • A light source
  • The cell you are sampling
  • A means of measuring the light the sample will absorb.

Once converted into a vapour, the sample is atomised. Then a beam of electromagnetic beam passes through it. The sample will absorb some of this radiation.

AAS measures the amount of light absorbed proportional to the number of atoms of the element being measured for.

There are two ways to atomise the sample:

  • Flame aspiration involves sucking a solution into a flame, where most of it is vapourised.
  • Electrothermal atomisation where the sample is placed in a hollow tube, and, once heated, is vapourised completely.

After vapourisation, the sample is ready for preparation for measurement.

Preparation of the sample requires weighing it then diluting it into a solution.

In most cases, vapourisation will have converted the sample into free atoms, regardless of its original chemical makeup.

The AAS process also requires a calibration curve, which will help determine the concentration of the element you are testing for based on previous measurements of it in known concentrations.

The measuring instrument, known as a spectrometer, is calibrated for the specified element. This calibration can be re-scaled, depending on how concentrated the sample is.

Once the sample is fed into the instrument, it will show up on the instrument’s calibration curve.

The normal light source used in AAS is a hollow cathode lamp. This type of lamp contains a hollow cathode made of the element being analysed, and an anode electrode.

Both these sealed in a hollow tube filled with a noble gas.

Gaseous ions bombard the cathode, which ejects metal ions. The cathode concentrates most of these emitted ions into a beam that passes through a quartz window.

Usually, atomic absorption spectrometers will have several different lamps for different elements.

With some elements, it is necessary to take precautionary steps to prevent contamination of readings from other atoms or molecules absorbing some of the light source in the background during the AAS process.

One method is to use two light sources, a cathode lamp and a deuterium lamp, which produces broad band radiation but not specific spectral lines.

By alternating measurements between the two lamps, the operator can subtract the amount of background absorption from the total readings, leaving only the figures needed for analysis.

The spectrometer also incorporates a monochromator. This is an optical device that selects and transmits a specified wavelength, or spectral line.

It selects the specific light appropriate to the element from the cathode lamp and directs it onto a detector.

This produces an electrical signal that is proportionate to the intensity of the light.

As a form of control, A double beam spectrometer will split the beam. One beam is for reference only, with the absorbance set at zero, while the other passes through the atom cell.

By constantly monitoring both the light source and the reference beam, you can ensure that the spectrum is not suffering a loss of sensitivity, as the intensity of the light source may not always stay constant.

What equipment is used to perform Atomic Absorption Spectroscopy?

There are various essential pieces of equipment necessary to performing atomic absorption spectroscopy:

  • Atomiser
  • Light source
  • Spectrometer
  • Detector

The spectrometer is, in effect, a system that incorporates all these elements.

It will, typically, include:

  • Furnace and flame
  • Spray chamber
  • Mirrors
  • Cathode lamp, and lamp carousel
  • Deuterium (D2) lamp
  • Beam selector
  • Photon detector

There are also various accessories to supplement the core AAS equipment, such as auto-dilutor systems for sample preparation and continuous flow vapour generation systems.

Because atoms to be analysed must be in a gas phase, the application of heat is essential to the process.

The Furnace used in AAS is made from graphite. It is in the form of a graphite tube. This generates the thermal energy to break the sample’s chemical bonds, producing free atoms for analysis.

There are three steps to this process:

  • Drying, which removes solvent from the sample
  • Ashing, which removes any organic and inorganic material
  • Atomisation, which generates the free atoms.
The optical path of an AAS spectrometers consists of a light source which emits at the wavelengths suitable for the element analysed. This light will be absorbed by the samples in equal proportion to the concentration of the analyte. The remaining light is received by a detector. The more light intensity has been absorbed, the higher the concentration of the analyte in the sample
The optical path of an AAS spectrometers consists of a light source which emits at the wavelengths suitable for the element analysed. This light will be absorbed by the samples in equal proportion to the concentration of the analyte. The remaining light is received by a detector. The more light intensity has been absorbed, the higher the concentration of the analyte in the sample

The flame, usually a slot-type burner, is used for analysing fluids. It vapourises them to create a gas.

The spray chamber introduces the sample, aspirated then applied as drops, into the flame.

The mirrors direct the light beams from the cathode and D2 lamps, and the beam selector splits the beam into component wavelengths.

The photon detector counts light in photons. Photons are elementary particles, the tiniest possible particle of light in an electromagnetic field.

Along with the equipment for performing AAS, the system requires the right kind of supporting software.

This software enables precise instrument control, and acquiring, manipulating and interpreting the data that the process generates.

What are the benefits of Atomic Absorption Spectroscopy?

Comparative sensitivity range and detection limit comparison of flame and graphite furnace AAS compared with ICP-OES and ICP-MS
Comparative sensitivity range and detection limit comparison of flame and graphite furnace AAS compared with ICP-OES and ICP-MS

As an analytical technique, AAS has several clear benefits:

  • It is accurate, typically producing results within a 0.5 per cent to 5 per cent range
  • As a method of detection, it is incredibly sensitive, measuring at parts per million (ppm)
  • It can analyse specific elements because of the unique light-absorbent qualities of their atoms
  • AAS can determine concentrations of over 65 elements.
  • It is a relatively simple process, drawing on well-documented protocols
  • AAS allows for a high throughput of samples
  • It is inexpensive in comparison to other analytical techniques.
  • AAS supports a broad range of industries and sectors, including environmental, chemical, petrochemical, food and drink and pharmaceutical.

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